1. Field of the Invention
The present invention relates generally to an active suspension system for an automotive vehicle. More specifically, the invention relates to an active suspension system with a suspension control system which can provides enhanced control characteristics at ON/OFF transition period of a power supply for assuring riding comfort upon re-starting of power supply short period after turning OFF.
2. Description of the Background Art
Japanese Patent First (unexamined) Publication (Tokkai) Heisei 1-122717 discloses one of typical construction of an active suspension system. The shown system includes a hydraulic cylinder disposed between a vehicular body and a vehicular wheel. A fluid pressure in the working chamber of the hydraulic cylinder is externally controlled by a hydraulic pressure circuit and a pressure control valve disposed in the pressure circuit. The shown system further has a power holding system for maintaining power supply for a given period after turning of a main switch of power source circuit. During the given period, the fluid pressure in the working chamber is gradually adjusted toward a predetermined neutral pressure. The system is further provided with a pressure holding mechanism for maintaining the fluid pressure in the working chamber at the neutral pressure while the power is not supplied to the control system. Namely, in the shown system, pressure control can be maintained in the given period for adjusting the fluid pressure in the working chamber at the neutral pressure. The control system is thereafter shut down with maintaining the pressure holding mechanism at the pressure holding position so as to maintain the pressure in the working chamber at the neutral pressure. The pressure holding mechanism includes a pilot operated operational one-way check valve which is designed to be maintained at shut-off position while the line pressure in the supply line of the pressure circuit is maintained lower than the neutral pressure. However, even in the modern technology, it is quite difficult to completely shut-off the pressure circuit and thus precisely maintain the fluid pressure in the working chamber at the neutral pressure. In other words, the fluid pressure can drop below the neutral pressure while the power supply is absent. Therefore, at the ON-set of the power supply to initiating pressure supply through the pressure circuit, the line pressure in the supply line is increased abruptly. However, since the operational check valve in the drain line is maintained shut-off position until the line pressure reaches the neutral pressure. As a result, at the initial stage of suspension control after resumption of power supply for the active suspension system, uncomfortable change of fluid pressure in the working chamber may be caused.